leds: Added driver for the NXP PCA9685 I2C chip

The NXP PCA9685 supports 16 channels/leds using a 12-bit PWM (4095
levels of brightness)
This driver supports configuration using platform_data.

Signed-off-by: Maximilian Güntner <maximilian.guentner@gmail.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>